1. Field of the Invention
This invention relates to ionically conductive polymer gel electrolytes and method of their preparation.
2. Description of the Prior Art
There has been considerable interest in recent years in developing ionically conductive polymer electrolytes that may be used in advanced electrochemical devices, such as fuel cells, lithium secondary batteries, supercapacitors and electrochromic windows. The goal has been to provide electrolytes with fast lithium ion transport at room temperature, combined with high chemical and electrochemical stability and good mechanical properties. Several electrolytes have been devised to meet this goal.
After the first reports of electrolytes based on polyethylene oxide (PEO) and propylene oxide (PPO), many different polymer electrolytes were developed and studied. These include poly(vinylidene fluoride) copolymer, poly(acrylo-nitrile), polyurethane, polyacrylamide, polytetrahydrofuran, poly(methyl methacrylate), poly(dimethyl siloxane) copolymers, etc. A characteristic feature of these solid polymer electrolytes (SPE) is the formation of a complex between a lithium salt (LiX) and a polymer having an electron donor atom such as O or N. The complex is stabilized by the X.sup.- anion, which usually has a large size. These SPE showed good electrochemical stability and perfect mechanical properties. However, the low values of ionic conductivity obtained at ambient temperatures (10.sup.-8 -10.sup.-5 S cm.sup.31 1 at room temperature) due to the glassy or semicrystalline state of the polymer structure is the major disadvantage of these electrolytes.
Other approaches have been proposed, such as the use of polymer blends, addition of plasticizers, reduction of the polymer content to very small amounts, or immobilization of liquid electrolyte solutions in polymer matrices. The latter approach appears to be the most promising, and more recently, work has been done on polymer electrolytes that are based on plasticized polymers. Typically these materials include a polymer, an ionic salt and a low molecular-weight organic solvent. A polymer/solvent mass ratio in gel electrolytes is usually less than 1/1 and the ionic conductivity reaches the value of 10.sup.-3 S cm.sup.-1.
Various procedures were used to immobilize liquid solutions in a polymer matrix, with the final goal of obtaining dimensionally stable membranes while still retaining liquid-like transport properties. These procedures are mainly based on light- or thermally induced crosslinking or gelification. However, the side effects of chemical crosslinking were found to almost completely destroy the quality of the polymer gels. For example, chemical impurities, free radicals, initiators, etc., reacted with and eroded the lithium anode. Further, after chemical crosslinking, the ionic mobility was found to decrease greatly and the ionic conductivity decreased to the order to 10.sub.-5 S/cm. Therefore, polymer gels with physical crosslink points stabilized by crystals or intermolecular entanglements are better candidates for electrolyte separators in lithium batteries, supercapacitors and electrochromic devices.
Physically crosslinked gel electrolytes have been the subject of a number of patent applications and have also been described in various publications.
For example, U.S. Pat. No. 5,219,679 issued to Abraham et al., discloses an electrolyte consisting of polyacrylonitrile, ethylene carbonate, propylene carbonate and lithium perchlorate, having a room temperature conductivity about 10.sup.-3 S cm.sup.-1. Another reference, U.S. Pat. No. 5,252,413 to Alamgir, et al., describes an electrolyte based on a polymer network comprising polyvinyl chloride, EC, PC and a lithium salt. The conductivity of this electrolyte reaches 1.4 10.sup.-3 S/cm at ambient temperature. Another patent, U.S. Pat. No. 5,639,574 issued to Hubbard, et al., discloses ionically conductive polymer gels comprising various polymers--poly (vinylidene fluoride), poly(ethylene terephthalate), Nylon 6,6 etc., an organic solvent, and a lithium salt, wherein the polymer is present up to 50% by mass, and ionic conductivity is greater than 1.times.10.sup.-3 S cm.sup.-1.
First systematic studies of PMMA based gel electrolytes, performed by Bohnke, et al., showed that atactic PMMA forms ionically conductive gels in solutions of lithium perchlorate in PC, or mixtures of PC and EC. At the polymer concentrations of about 20% by weight, these gels provided room temperature conductivity in the range of 10.sup.-3 S/cm, and exhibited elasticity at higher frequencies of the dynamic mechanical testing.
Following these studies, there have been reported a number of gel electrolytes, that were based on PMMA. For example, a formulation comprising atactic PMMA, PC, EC and LiCIO.sub.4 (25:35:35:5) is described by L.Su, et al., as a gel electrolyte for an electrochromic display device [L.Su, J. Fang, Z Xiao and Z.Lu, Thin Solid Films 306,133-136 (1997)].
Another example describes a composition consisting of PMMA, PC, EC and LiCIO.sub.4 (30:19:46.5:4.5) as a gel electrolyte for a lithium metal-polypyrrole secondary cell [T. Osaka, T. Momma, H. Ito and B. Scrosati, Journ. Power Sources 68,392-396 (1997)].
Yet another reference (Japanese Patent Specification No. 5586070), discloses a battery electrolyte comprising PMMA, PC or .gamma.-butyronitrile, and LiBF.sub.4 in the form of a gel possessing elasticity and having conductivity in the range of 10.sup.-3 S cm.sup.-1.
Other pertinent publications include: L.Su, Z. Xiao, Z. Lu, "All solid-state electrochromic device with PMMA gel electrolyte," Materials Chemistry and Physics 52,180-183 (1998), W. Zhu, S. Zhang, Z. Huang, "Characteristic of ion conductive electrode based on poly(methyl methacrylate), Dianyuan Jishu 21(6), 248-251 (1997), X. Yang, C.Li, G. Chen, "Polymeric gel electrolyte and its application to electrochromic smart window," Yingyong Huaxue, 14(5), 59-62 (1997), Tetsuya Osaka, Toshiyuki Momma, Hidetoshi Ito, Bruno Scrosati, "Performances of lithium/gel electrolyte/polypyrrole secondary batteries," J. Power Sources, 68(2), 392-396 (1997), X. Liu, T. Osaka, "Properties of electric double-layer capacitors with various polymer gel electrolytes," J. Electrochem. Soc., 144 (5), 3066-3071(1997), T. Osaka, T. Momma, H. Ito, B. Scrosati, "Cyclability of lithium/poly(methyl methacrylate)-based gel electrolyte/polypryrrole battery," Proc.--Electrochem. Seoc. 96-17,1-3(1997), X. Liu, T. Osaka, "Properties of electric double-layer capacitors with various polymer gel electrolyte/polypyrrole battery," Proc. -Electrochem.Soc. 96-17, 1-3 (1997), P. E. Stallworth, S. G. Greenbaum, F. Croce, S. Slane and M. Salomon, "Lithium-7 and ionic conductivity studies of gel electrolytes based on poly (methyl methacrylate)," Electrochim. Acta 40 (13-14), 2137-2144 (1995), M. Rezrazi, M. Mullet, O. Bohnke, "Conductivity and viscosity studies of lithium ion conductive electrolytes gelled with poly(methyl methacrylate)," Adv.Mater.Res (Zug.Switz.) 1-2, 495-499 (1994).
The most recent patent reference (U.S. Pat. No. 5,581,394 to Green, et al.) discloses a solid polymer electrolyte for electrochromic devices based on PMMA, PC and LiCIO.sub.4. In order to solidify the electrolyte it is proposed to partially remove the solvent--PC after preparation of the PMMA/PC/LiCIO.sub.4 composition, so that the final concentration of the polymer is about 40%. The conductivity of such quasi-solid electrolytes is less than 10.sup.-4 S Cm.sup.-1.
The obvious drawback of these systems is their poor mechanical properties, and as is evident from the last example, their mechanical properties can only be improved at the expense of conductivity decreases by increasing the polymer/solvent ratio. Another problem with electrolytes based on PMMA/PC/lithium salt, is that the solvent PC, because of its high vapor pressure, easily corrodes the lithium metal anode in an electrochemical cell. However, high molecular weight atactic PMMA does not form gels with more stable solvents such as EC, DMC, DEC, or their mixtures.
To overcome the shortcomings of existing PMMA gel electrolytes, an alternative gel electrolyte is provided.
It has been known for a long time that isotactic and syndiotactic PMMA can form stereocomplexes in solutions and in bulk. W. Borchard, M. Pyrlik and G. Rehage "Association Phenomena of PMMA in Solutions and Gels," Die Makromelukulare Chemie 145, (1971) 169-188. The stereo-association between i-PMMA and s-PMMA has been studied in a number of complexing solvents, such as toluene, DMF, THF, acetone, benzene, etc. A drastic (approximately 25 times) increase in viscosity and a weak gel formation was observed in dilute polymer solutions (about several weight percent) when the ratio of isotactic to syndiotactic fractions in the mixture was 1 to 1.
The mechanical properties of PMMA gels were thought to be capable of enhancement by stereocomplexation and chain entanglement between i-PMMA and s-PMMA. Although s-PMMA is not commercially available, it is known that commercial atactic PMMA does not contain a really random distribution of isotactic and syndiotactic dyads, but rather contains long regions of syndiotactic sequences. Therefore, it seemed reasonable to test experimentally whether (i) the stereo association between i-PMMA and syndiotactic sequences of a-PMMA will occur in solutions of lithium salts in propylene carbonate and other alkaline carbonates employed as solvents for lithium salt electrolytes; (ii) these stereo complexes pervade the whole system like crosslinks, forming three dimensional networks and providing additional rigidity.
The gel electrolytes of the invention do not suffer from the problems of the prior art and provide many positive advantages.